But Amazon doesn't make any money on their devices, so they say, so they shouldn't be included.

I'm not sure that's a valid argument.

Both Amazon and Apple purport to have a wide ecosystem of books, movies, music and apps available from their devices.

The fact that Amazon choose to make their profit purely from the content doesn't alter the fact that from the consumer's point of view their devices are cheaper.

But, yes, this may well be the key difference, as demonstrated by the iPad mini beating the Samsung 7.7 on price (where although the 7.7 has better specs overall they have very similar screen sizes, batteries and build quality).